A. Field of the Invention
The invention relates generally to read-only memories for microprocessor based CPU modules and more particularly to hierarchied auxiliary ROM memory systems for additions and alterations to on-board ROM.
B. Description of the Prior Art
Previously, read-only memories for a microprocessor based CPU module could only be on-board mask programmable ROMS and/or relatively expensive field programmable PROMS. If an addition or alteration was needed to be made to the application program stored in the on-board memory, the whole board would have to be pulled and discarded in the case of mask programmable ROMS, or alternatively additions and alterations to code could be had only in a limited manner according to the amount of unused existing on-board memory in the case of PROMS. The former was expensive due to the microprocessor peripheral content and the latter equally unacceptable due to the large amount of highly skilled labor required to carefully pull, reprogram, and replace the CPU module board containing the ROM memory.
Even in those cases where there exists a rudimentary external memory board that has ROMS and/or PROMS that may be coupled through a system bus to the CPU module board, there still remains the problem of how to address, in a coherent hierarchal manner, the various categories of memory used for extension, insertion, large-scale overlay, and small-scale overlay while still retaining the flexibility required to increment and/or decrement the memory population as the need arises.